The object of the proposed research is to develop optical probes (dyes that absorb and fluoresce visible light) that can routinely be used by neurophysiologists to measure membrane potentials in nerve and other excitable cells too small to permit the use of microelectrodes. We plan to uncover the mechanism by which these probes sense membrane potentials so that (1) the size of the optical signal can be calibrated as a function of membrane voltage, (2) we can use our knowledge of the mechanism to design and synthesize more sensitive probes, and (3) we can use the probes to study the structure and dynamics of nerve membrane. We intend to accomplish our objectives by correlating the physical and spectral properties of the probes with the size, direction, and time course of their fluorescence of absorbance responses during voltage clamped potential changes in the squid giant axon (to which the probes are added). We will optimize the sensitivity of the probes to membrane potential by intelligent chemical modification of their structures. The voltage dependent absorption and fluorescence properties of our dyes will be studied with an optical black lipid membrane apparatus in order to elucidate the mechanism of membrane potential sensitivity. Quantitative models will be developed to describe the fluorescence or absorption signal as a function of membrane potential.